1. Field of the Invention
The present invention relates to a two-way optical space communication apparatus provided with a function to adjust an angle of transmitting light or receiving light, which performs two-way (bidirectional) information transmission by propagating a light signal in a beam shape in a free space.
2. Related Background Art
In conventional optical space communication apparatus, a transmission-side device transmits a signal in which a pilot signal is superimposed on a main signal (transmission signal), and a reception-side device detects the pilot signal to detect an angular deviation between the optical axis of a receiving optical system and an arrival direction of the receiving light, thereby adjusting the angle upon start of operation or correcting the angle during operation. The pilot signal is a signal having a frequency band different from and narrower than that of the main signal. An example of the pilot signal is a sinusoidal signal.
A generally employed method for detecting the angular deviation is one arranged in such a manner that a receiving light spot focused by the receiving optical system is guided onto a photodetector and the photodetector detects a position of the spot. The photodetector employed is a PSD (Position Sensitive Detector) or a CCD (Charge Coupled Device), which demonstrates weakness with respect to the response speed when the frequency of the pilot signal is high. For overcoming it, generally employed is a method using an array of plural photodetectors comprised of photodiodes with fast response speed and detecting differences of outputs from the photodiodes, in which the photodetectors with same characteristics are located in the four quadrants and the position of the receiving light spot is obtained from a sum and a difference between the outputs from the photodetectors.
The reason why the pilot signal is used is that high-sensitive reception is possible because of its narrower band than that of the main signal and that the control function can be maintained even if the main signal becomes weak or even if there is no input of the main signal. Further, influence of background light can be decreased when the angular deviation is detected with the pilot signal having a certain high frequency component, different from low frequency components such as dc light.
The background light is all light other than the light transmitted from the transmission-side device, in the light entering the reception-side device. For example, the background light includes sunlight, lights of buildings, etc.
However, when the background light greatly increases, for example if the direct rays of the sun are incident on the light receiving elements for detecting the main signal and angular deviation signal, these light receiving elements could be damaged in the worst case. There is another problem that the angular deviation detector fails to discriminate the desired receiving light from the background light so as to erroneously correct the angle with respect to the arrival direction of the background light, which results in failing to recognizing the arrival direction of the desired receiving light even after the background light decreased, thereby failing to returning to the normal communication condition.